Thermal spraying of stainless steel

ABSTRACT

A method of preparing a roughened substrate surface for subsequent coating with a fluorocarbon polymer in a liquid medium. The surface of the substrate is thermally sprayed with a stainless steel alloy containing from 25 to 35 percent by weight of chromium. A metal-coated substrate produced by this method and an article having thereon a fluorocarbon surface coated by this method are also described.

This invention relates to the thermal spraying of stainless steel,especially as applied as an intermediate stage in the provision of anon-stick coating layer over a substrate metal.

It is well known to spray on to a roughened substrate surface afluoropolymer dispersion, to remove the dispersant or solvent, and tocure or sinter so as to give a coating layer.

In order to improve the adhesion of this layer to the substrate it isalso well known first to flame-spray metal droplets or particles on thesubstrate so as to give a rough surface for the fluorocarbon polymeradhesion. An example of such flame sprayed metal is stainless steel,which is a general name for alloys of iron chromium, (typically about18%) nickel (typically about 8%) and minor additives such as titanium,manganese, silicon etc. However, it is known in the art that problems ofcoating breakdown can arise and it is recognised generally that many ofthese have their roots in corrosion phenomena as between the substrateand the flame-sprayed metal

For example, it is accepted that aluminium is not a very suitablesubstrate metal for such techniques and this is believed to be becauseof the risk of "white rust" corrosion, that is to say formation ofaluminium oxides within the coating layers.

The present invention has originated from a consideration of thisproblem and involves utilisation of a form of stainless steel with anunusually high chromium level and optionally with increased nickellevel.

On an aluminium substrate this runs completely contrary to previousexpectations, and the use of additional chromium is counter-indicatednot only by cost but also by the expectation of increased possibility ofwhite rust (corrosion due to aluminium salts). due to increased chromiumcontent. However, and as discussed in detail below, we believe we haveidentified a different corrosion problem as being more significantalthough hitherto not appreciated, and we can alleviate that problem toobtain improved coatings, any risk of "white rust" formation being morethan offset by curing this other problem having its origin in "red rust"(corrosion due to ferrous salts) formation.

In one aspect the invention consists in a method of preparing aroughened surface for subsequent coating with a fluoro polymer in aliquid medium, in which method the surface is thermally sprayed (e.g.flame sprayed or sprayed on using an electric arc or plasma gun) with astainless steel alloy the chromium content of which is increased overthat of standard stainless steel.

Preferably the chromium content of the stainless steel, being greaterthan the normal 18% of standard stainless steel, ranges up to 35%. Mostpreferably it lies within the range of 25 to 35% by weight.

Preferably also there is some small adjustment in the nickel content ofthe stainless steel which can be up to 15% that is to say rather greaterthan the normal 8% figure for standard stainless steel.

A preferred composition of stainless steel for carrying out the methodof the invention is therefore 25 to 35% by weight of chromium, 8 to 15%by weight of nickel. 0.1% maximum by weight of carbon 2% maximum byweight of manganese, and 0.4% maximum by weight of silicon, with theremainder being iron and incidental impurities.

The method as defined above is typically carried out to provide a roughsurface with protruding particles. These particles can occupy, formaximum roughness, somewhere from 50 to 85% of the total area of thesubstrate to be eventually coated. The roughness of the particlesthemselves is preferably from 25 to 50 % microns transverse dimension,most preferably by way of example 32 microns.

Typically, the substrate surface itself has previously been roughenedbefore being thermally sprayed. Such previous roughening can typicallyhave the dimensions of from 15 to 20 microns peak-to-valley distance.

In the method of the invention as defined above a wide range ofsubstrate metals can be used but it is a particular advantage of theinvention that even the more difficult substrate metals such asaluminium may be utilised.

The invention also extends to the metal coated substrate produced by theabove method. In particular, however, the invention further extends tothe above method combined with a subsequent coating stage withfluorocarbon polymer, for example coating the fluorocarbon polymer fromliquid media (dispersant or solvent vehicle) and subsequent drying (e.g.at 120° C. to 150° C.) and curing or sintering stages (e.g. at 200-450°C., for up to 30 minutes) which together achieve a non-stick anddurable, corrosion resistant, layer.

The invention of course further extends to an article having a surfacecomprising such a fluorocarbon layer secured to an intermediatecorrosion-resistant layer as described above. The article in questioncan be a tool (secateurs, shears, fork).

The above invention using an excess of chromium in the thermally-sprayedalloy, is not predictable from prior art, especially in the context ofan aluminium substrate where the use of excess chromium might beexpected to give even more aluminium oxides. However, although we do notwish to be governed by any theory as to the operation of the method, itappears likely that the phenomenon of red rust, previously unexpected instainless steel is in fact present, and although not always immediatelynoticeable, persists over a long period. It would appear to have itsorigin in the drastic thermal spraying conditions apparently leading toat least superficial loss of chromium in at least some of the metalparticles probably dependent upon their individual history as they passdifferently through different temperature zones.

The following observations appear to support the above hypothesis.

1. Glass panels metal sprayed with traditional stainless steel grades ofwire produced red corrosion products in neutral salt fogs (ASTM B4117),static 5% sodium chloride solutions and static 10% acetic acid solutionswithin a 25 hour exposure duration.

2. Aluminium plaques (of both cast LM6 and wrought 1200 grades) metalsprayed with traditional stainless steel grades of wire produced bothred and white corrosion products in boiling 10 % acetic acid solution insome cases after only 20 minutes exposure.

3. Electron microprobe analysis of the metal deposited on to panelsshowed a cored structure, consisting of a very thin surface layercontaining approximately 14% chromium and in some cases the surprisingsituation of a bulk central region with virtually no chromium present.The other alloy elements were evenly distributed.

The non-staining properties of stainless steel are due to the formationof a tenacious and resistant oxide film based on chromium. It isgenerally regarded that chromium levels below 14-15% are insufficient toimpact protection and the structure of the deposited layer is thereforesuch that the iron oxide film formed is extremely thin. Any abrasion islikely to break the film to expose the central core Insufficientchromium is present to heal the break in the film and corrosion proceedsunabated.

4. Aluminium vessels which had been metal sprayed using traditionalstainless steel wire were used to boil a 10% acetic acid solution for 3hours. In all cases a red discolouration was found, allied sometimeswith areas of white corrosion products. In an effort to identify the redproduct a pan was left standing for 30 minutes in a 10% oxalic acidsolution. After this time the acid was discoloured a nd the pan restoredto its original metallic appearance. Polarographic analysis of the acidsolution indicated the pressure of Fe² + and Fe³ ions. Another similarlytreated Pan was filled with deionized water and was still discolouredeven after a 2 hour boil.

5. Panels of typical cookware compositions such as cast or wroughtaluminium stainless steel or cast iron when coated with the specifiedwire in the prescribed manner have not shown and tendency to for eitherred or white corrosion products in typical cooking environments such asboiling salted water, hot detergents, salty steam and boiling aceticacid solutions after a minimum three hour exposure.

I claim:
 1. A method of preparing a roughened surface for subsequentcoating with a fluoropolymer in a liquid medium, in which method saidsurface is thermally sprayed with a stainless steel alloy wherein saidstainless steel contains from 25 to 35% by weight of chromium.
 2. Amethod according to claim 1, wherein said stainless steel contains from8 to 15% by weight of nickel.
 3. A method according to claim 1, whereinsaid stainless steel further comprises 8 to 15% by weight of nickel,0.1% maximum weight of carbon, 2% maximum by weight of manganese and0.4% by weight of silicon, the remainder being iron and incidentalimpurities.
 4. A method according to claim 1, arranged to cover from 50to 85% by area of said surface.
 5. A method according to claim 4,wherein said thermally sprayed alloy comprises particles which rangefrom 25 to 50 microns in their transverse dimension.
 6. A methodaccording to claim 1, wherein said surface is itself pre-roughened,prior to said thermal spraying.
 7. A method according to claim 6,wherein the peak-to-valley distance of said pre-roughened surface isfrom 15 to 20 microns.
 8. A method according to claim 1, wherein saidsurface is an aluminum surface.
 9. A metal-coated substrate produced bythe method defined in claim
 1. 10. A metal-coated substrate having aroughened surface for subsequent coating with a fluoropolymer in aliquid medium, comprising:a substrate, and a coating formed by thermallyspraying an alloy having from 25% to 35% by weight of chromium and from8% to 15% by weight of nickel on said substrate, said coating formingsaid roughened surface on said substrate.
 11. A metal-coated substrateaccording to claim 10, wherein said alloy further includes 0.1% maximumby weight of carbon, 2% maximum by weight of manganese and 0.4% maximumby weight of silicon, the remainder being iron and incidentalimpurities.
 12. A metal-coated substrate according to claim 10, in whichsaid thermally sprayed alloy covers from 50 to 85% of the area of thesubstrate to be subsequently coated with said fluoropolymer, and whereinsaid thermally sprayed alloy comprises particles which range from 25 to50 microns in their transverse dimension.
 13. A metal-coated substrateaccording to claim 10, in which said surface is pre-roughened prior tosaid thermal spraying, and wherein the peak-to-valley distance of saidpreroughened surface is from 15 to 20 microns.
 14. A metalcoated-substrate according to claim 10, wherein said surface is analuminum surface.
 15. A method of coating a substrate with afluorocarbon layer, comprising:preparing a roughened surface on saidsubstrate by thermally spraying a stainless steel alloy thereon, saidstainless steel alloy containing from 25% to 35% by weight of chromium;coating the metal-coated substrate with a fluorocarbon layer from liquidmedium; drying said coated substrate; and curing or sintering saidsubstrate.
 16. A method according to claim 15, wherein said drying iscarried out at 120° to 150° C., and said curing or sintering is carriedout at 200° to 450° C. for up to about 30 minutes.
 17. An article havingthereon a fluorocarbon surface coated by the method as defined in claim15.
 18. An article according to claim 15, wherein said article is in theform of a tool.